The use of natural products as starting materials for the manufacture of various large-scale chemical and fuel products which are presently made from petroleum- or fossil fuel-based starting materials, or for the manufacture of biobased equivalents or analogs thereto, has been an area of increasing importance. For example, a great deal of research has been conducted into the conversion of natural products into fuels, as a cleaner and, certainly, as a more sustainable alternative to fossil-fuel based energy sources.
Agricultural raw materials such as starch, cellulose, sucrose or inulin are inexpensive and renewable starting materials for the manufacture of hexoses, such as glucose and fructose. It has long been appreciated in turn that glucose and other hexoses, in particular fructose, may be converted into other useful materials, such as 2-hydroxymethyl-5-furfuraldehyde, also known as 5-hydroxymethylfurfural or simply hydroxymethylfurfural (HMF):
The sheer abundance of biomass carbohydrates available provides a strong renewable resource base for the development of commodity chemical and fuel products based on HMF. For example, U.S. Pat. No. 7,385,081, issued in June 2008 to Gong, estimates, for example, that of the approximately 200 billion tons of biomass produced annually, 95% was in the form of carbohydrates, and only 3 to 4% of the total carbohydrates were then used for food and other purposes.
In view of this fact, and due to HMF's various functionalities, it has been proposed that the HMF thus obtainable from hexoses such as fructose and glucose, could be utilized to produce a wide range of products derived from renewable resources, such as polymers, solvents, surfactants, pharmaceuticals, and plant protection agents. HMF has in this regard been proposed, as either a starting material or intermediate, in the synthesis of a wide variety of compounds, such as furfuryl dialcohols, dialdehydes, esters, ethers, halides and carboxylic acids.
A number of the products discussed in the literature derive from the oxidation of HMF. Included are hydroxymethylfurancarboxylic acid (HmFCA), formylfurancarboxylic acid (FFCA), 2,5-furandicarboxylic acid (FDCA, also known as dehydromucic acid), and diformylfuran (DFF). Of these, FDCA has been discussed as a biobased, renewable substitute in the production of such multi-megaton polyester polymers as poly(ethylene terephthalate) or poly(butylene terephthalate). Derivatives such as FDCA can be made from 2,5-dihydroxymethylfuran and 2,5-bis(hydroxymethyl)tetrahydrofuran and used to make polyester polymers. FDCA esters have also recently been evaluated as replacements for phthalate plasticizers for PVC, see, e.g., WO 2011/023491A1 and WO 2011/023590A1, both assigned to Evonik Oxeno GmbH, as well as R. D. Sanderson et al., Journal of Appl. Pol. Sci. 1994, vol. 53, pp. 1785-1793.
While FDCA and its derivatives have attracted a great deal of recent commercial interest, with FDCA being identified, for instance, by the United States Department of Energy in a 2004 study as one of 12 priority chemicals for establishing the “green” chemical industry of the future, the potential of FDCA (due to its structural similarity to terephthalic acid) to be used in making polyesters has been recognized at least as early as 1946, see GB 621,971 to Drewitt et al, “Improvements in Polymer”.
Unfortunately, while HMF and its oxidation-based derivatives such as FDCA have thus long been considered as promising biobased starting materials, intermediates and final products for a variety of applications, viable commercial-scale processes have proven elusive. Acid-based dehydration methods have long been known for making HMF, being used at least as of 1895 to prepare HMF from levulose (Dull, Chem. Ztg., 19, 216) and from sucrose (Kiermayer, Chem. Ztg., 19, 1003). However, these initial syntheses were not practical methods for producing HMF due to low conversion of the starting material to product. Inexpensive inorganic acids such as H2SO4, H3PO4, and HCl have been used, but these are used in solution and are difficult to recycle. In order to avoid the regeneration and disposal problems, solid sulfonic acid catalysts have also been used. The solid acid resins have not proven entirely successful as alternatives, however, because of the formation of deactivating humin polymers on the surface of the resins. Still other acid-catalyzed methods for forming HMF from hexose carbohydrates are described in Zhao et al., Science, Jun. 15, 2007, No. 316, pp. 1597-1600 and in Bicker et al., Green Chemistry, 2003, no. 5, pp. 280-284. In Zhao et al., hexoses are treated with a metal salt such as chromium (II) chloride in the presence of an ionic liquid, at 100 degrees Celsius for three hours to result in a 70% yield of HMF, whereas in Bicker et al., sugars are dehydrocyclized to HMF at nearly 70% reported selectivity by the action of sub-or super-critical acetone and a sulfuric acid catalyst.
In the acid-based dehydration methods, additional complications arise from the rehydration of HMF, which yields by-products such as levulinic and formic acids. Another unwanted side reaction includes the polymerization of HMF and/or fructose resulting in humin polymers, which are solid waste products and act as catalyst poisons where solid acid resin catalysts are employed, as just mentioned. Further complications may arise as a result of solvent selection. Water is easy to dispose of and dissolves fructose, but unfortunately, low selectivity and the formation of polymers and humin increases under aqueous conditions.
In consideration of these difficulties and in further consideration of previous efforts toward a commercially viable process for making HMF, Sanborn et al. in US Published Patent Application 2009/0156841A1 (Sanborn et al) describe a method for producing “substantially pure” HMF by heating a carbohydrate starting material (preferably fructose) in a solvent in a column, continuously flowing the heated carbohydrate and solvent through a solid phase catalyst (preferably an acidic ion exchange resin) and using differences in the elution rates of HMF and the other constituents of the product mixture to recover a “substantially pure” HMF product, where “substantially pure” is described as meaning a purity of about 70% or greater, optionally about 80% or greater, or about 90% or greater. An alternative method for producing HMF esters performs the conversion in the presence of an organic acid, which can also serve as the solvent. Acetic acid is mentioned in particular as a solvent for fructose. The resulting acetylated HMF product is reported to be “more stable” than HMF, because upon heating HMF is described as decomposing and producing byproducts “that are not easily isolated or removed,” page 4, paragraph 0048.
Further, the acetylated HMF is said to be more easily recovered by distillation or by extraction, though filtration, evaporation and combinations of methods for isolating the HMF esters are also described (page 2, para. 0017). The product, HMF ester which may include some residual HMF, can then be mixed in one embodiment with organic acid, cobalt acetate, manganese acetate and sodium bromide and oxidized to FDCA in the presence of oxygen and at elevated temperatures and pressures. In the examples, a Parr reactor is used for performing the oxidation.
Those familiar with the manufacture of terephthalic acid will appreciate the fact that the same Co/Mn/Br catalyst system conventionally used in the Mid-Century Process, for liquid-phase oxidation of para-xylene to terephthalic acid, was thus shown to be useful in the oxidation of HMF esters and residual HMF to TPA's biobased analog FDCA. The capacity to source and use, for converting biobased materials, the same catalyst as used predominantly in the processing of petroleum-derived materials is a valuable and desirable feature.
Very recently published WO 2011/043661 (hereinafter, “WO'661”) describes continuing efforts to produce FDCA commercially from carbohydrates such as fructose and glucose through HMF and HMF derivatives as intermediates. After summarizing their view or interpretation of previously published methods for the oxidation of HMF to FDCA in an aqueous medium using a Pt-group catalyst or involving the oxidation of HMF over a gold-based catalyst, the inventors in WO'661 contend that Sanborn et al. failed in fact to produce FDCA from the 5-(acetoxymethyl)furfural (AMF) ester formed through the reaction of HMF with the acetic acid solvent. “Surprisingly” the inventors in WO'661 find that when using an oxidation catalyst based on cobalt and manganese and containing a bromide, various furan-based materials inclusive of 5-(acetoxymethyl)furfural and other like ester derivatives of HMF can provide FDCA in “high yields” provided reaction temperatures higher than 140 degrees Celsius are employed.
The HMF ester starting materials common to both Sanborn et al. and WO'661 are indicated in WO'661 as proceeding from known methods, wherein a carbohydrate source is converted in the presence of an alkyl carboxylic acid into products comprising an HMF ester and optionally HMF. Then an HMF ester and optional HMF feed is isolated from the products for subsequent oxidation at the greater than 140 degree Fahrenheit, alleged critical temperatures. While batch, semi-continuous and continuous processes are contemplated generally, “operation in the batch mode with increasing temperature at specific times, increasing pressure at specific times, variation of the catalyst concentration at the beginning of the reaction, and variation of the catalyst composition during the reaction” is indicated as preferred (pg 4, lines 28-32). And, while the pressure in the oxidation process of WO'661 is expressly observed to be dependent on the solvent pressure, page 4, last line to page 5, line 1, the preference is that the pressure should be such that the solvent is “mainly in the liquid phase”, page 5, line 2.